What is DeFi decentralized finance?

Decentralized Finance (DeFi) is the movement that leverages decentralized networks to transform old financial products into trustless and transparent protocols that run without intermediaries.

Some may prefer to call DeFi as Open Finance, Distributed Finance, or EthFi(referring to the fact that at the moment most of existing DeFi products built on Ethereum.)

Also most likely we can count DeFi as a subset of Fintech.

Here is a great overview of DeFi core components:

I'd say that absolutely needed components for DeFi are:

Smart contracts enabled network itself
Stablecoins
Liquidity on DEXes, available for integrations with DeFi products
And most important is mature enough network-wide ecosystem, with dev tooling and users who could use those DeFi dApps.

Check defiprime.com for a list of defi products and defipulse.com for live stats.